The present invention relates to an optical transducer for detecting the angular position of a rotating member, with respect to a fixed structure. A use of this arrangement, which is given by way of example and without limitation, is in relation to the shaft of a motor which provides for selection of the character of a character-carrying member of an office printing machine or movement of the character-carrying member with respect to a line of print.
The optical transducer according to the present invention comprises a disc which is connected to the rotating member and which is provided with a plurality of radial openings, a light emitting means and means for detecting the passage of the light through the openings of the disc.
A transducer is known, wherein the light generated by a first emitting means is divided into a plurality of light beams by a fixed mask which is arranged between the light emitting means and the apertured disc, and on which a second series of openings is provided. The light detecting means comprise a plurality of phototransistors, each of which co-operates with one of the light beams produced by the mask, to generate cyclic electrical signals which are out-of-phase with each other and which, when suitably processed, are indicative of the angular position of the rotating member and the direction of rotation thereof. A second light emitting means co-operates with a synchronisation opening of the disc and with a corresponding light detecting means for determining a reference or zero position of the rotating member. In this transducer, the light from the first light emitting means is divided by the mask and produces light beams which are slightly divergent relative to each other, whereby any axial movement of the disc with respect to the fixed mask causes a variation in the amount of light which passes through the openings in the disc and which reaches the detecting means, which in consequence can generate signals which are distorted and out of phase with respect to the true position of the rotating member.